I haven't read the whole thread..
But tbh I don't think it's black and white - and I speak as someone with a disability although I am not in a wheelchair.
While I appreciate that we should have consideration for others I think this goes both ways and that no one person deserves more consideration than anyone else purely based on their circumstances. Yes sometimes we will do things to benefit someone else but this should not be an automatic expectation - we are all individuals and all have individual lives and needs and circumstances and one does not necessarily take precedence over the other.
I also think it is extremely patronising to make the blanket statement/assumption that because someone has a disability they necessarily have a more difficult life - this is absolutely not the case. Someone with a disability may have to do things slightly differently to someone without but this does not automatically equal a hard life.
My disability precludes me from being able to drive a car/do certain jobs/I need certain technology to make the computer accessible/I need either a guide dog or a cane in order to have full mobility outside. But I certainly do not consider that my life is any more difficult than anyone else's based on the fact I happen to be considered to have a disability, and I hate this inference that it is. In fact to make that assumption still puts the disabled on an unequal footing with the able-bodied where able-bodied equals lucky/should be grateful/should make allowances, and the disabled equals unlucky/has a hard life/is deserving of favoured treatment on the basis of disability.
To the point in the op, perhaps if everyone had been queueing to get in the lift I could see that people might be more inclined to allow the wheelchair user in first, although even then I certainly don't think it is an entitlement, but I can see that people might think more along those lines. However, given the people in the lift were already there, had already started their journey on another floor, I certainly don't think it should be an automatic assumption to get out halfway to where you're going and allow the wheelchair user in.
Those people were in a lift for a reason; their need to be there is not any less justified based on the fact they could take the stairs.
And yes, the wheelchair user could wait for the next lift. And yes, the next lift might be full also - it is highly unlikely that every lift for the rest of the day is going to be full - at best he might need to wait a few minutes, in the same way those expected to get out would need to take a few extra minutes to get where they were going. Sometimes I have to cross busy main roads. Sometimes it is difficult because of the sheer amount of traffic. Sometimes cars stop for me, and sometimes they don't - which means that I have to wait an extra few minutes to cross the road. I don't have an alternative - I have to cross that road. But that doesn't mean that any car driving down it should stop for me just in case the next one doesn't and I have to wait a few more minutes to do so, although I am of course always very grateful when they do.
As for those on the thread saying that they know someone in a wheelchair who would ram the lift/wished they had a cattle-prod on the front for people like that - what a bloody entitled attitude - they might want to be careful not to overbalance under the weight of that chip on their shoulder. 
Having a disability does not make you any more entitled any more than not having a disability makes you ignorant/rude/selfish if you don't always put the perceived needs of the disabled first..